This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-173023, filed Jun. 18, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a radiation detector mounted in an X-ray CT (Computed Tomography) apparatus having the function of acquiring multidirectional projection data and the function of reconstructing a tomographic image based on the acquired data, and the X-ray CT apparatus.
Recently, as an X-ray CT radiation detector, a detector using solid-state detection elements using scintillators or X-ray detection elements has been developed. According to the detection principle of this device, as is known, the energy that X-rays lose due to ionization in a depletion layer formed upon application of a reverse bias to the rectifier junction of a diode is extracted as a current. Since energy conversion from X-rays to electric charges is direct and efficient, the energy resolution is very high, and the spatial resolution is also high. For this reason, this device is expected to be used widely in the future. In general, a one- or two-dimensional array of these solid-state detection elements is mounted on the rotating base of a gantry while being housed in an optically sealed case.
This solid-state detection element generally has temperature dependence, so that a temperature environment in which the device exhibits most efficient conversion efficiency is produced by using a heat panel. More specifically, a heat panel is mounted on the outside of the case to heat the overall case by using high thermal conductivity of the case consisting of aluminum or the like.
In general, several thousand channels are prepared for the X-ray CT radiation detector. If one solid-state detection element is used for one channel, several thousand solid-state detection elements are required. In this case, a problem arises in terms of temperature nonuniformity. Even if temperature dependence is made uniform among solid-state detection elements, variations in environmental temperature for each element will produce errors in output signals, resulting in artifacts (false images). The problem of temperature nonuniformity becomes increasingly significant owing to a high cooling effect produced as the gantry rotates at high speed for the purpose of shortening the scan time.
More specifically, a solid-state detection element array and collimator are mounted in an optically sealed cast case, and the temperature in the case is kept constant by using a heat panel stuck on the outer surface of the case and temperature control. From the viewpoint of the flow of heat, the heat generated by the heat panel is transferred to the overall case and base and spreads to the overall radiation detector. During the rotation of the gantry, however, the temperature of the head portion of the case, in particular, extremely drops due to a cooling effect. On the other hand, if the heat value of the heat panel is increased to suppress the cooling of the head portion, the portion other than the head portion is excessively heated. Currently, attempts to solve this problem have been repeatedly made on a trial-and-error basis in the process of selecting materials for a case and designing its shape. However, no satisfactory results have been obtained. In addition, this method undesirably limits the degree of freedom in case design, and hence cannot flexibly cope with changes in size, weight, cost, and the like.
It is an object of the present invention to provide a radiation detector and X-ray CT apparatus which can make internal temperature more uniform with a relatively simple structure.
A radiation detector for X-ray CT includes a heat insulating case mounted on a rotating base, a radiation incident window formed in the heat insulating case, a detection panel on which detection elements for detecting radiation incident through the radiation incident window are arrayed. The detection panel is positioned to form a fluid circulatory path in the heat insulating case. A circulator for circulating the fluid is placed in the circulatory path. The circulatory path and circulation of the fluid by the circulator make it possible to make the temperature of the detection panel relatively uniform.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.